The latest Marquette Law School poll of the Wisconsin Senate race shows Rep. Tammy Baldwin (D-Wis.) leading former governor Tommy Thompson (R) , 50 percent to 41 percent, a sharp turnabout from a mid-August survey that showed the Republican ahead by the same margin.

(J. Scott Applewhite/AP)

It's the most recent (and perhaps most dramatic) survey released this week with good news for Senate Democrats, who have shored up their standing in several battlegrounds.

In Wisconsin, the well-known Thompson's mid-August victory in a bruising Republican primary was a boon to the GOP's chances of picking up the seat. But in the weeks since he won, things haven't been so rosy. A Quinnipiac University/New York Times/CBS News poll released Wednesday showed Baldwin tied with Thompson, after trailing by 6 points last month. Baldwin released a poll of her own conducted last week that showed her leading slightly.

The primary left Thompson's war chest depleted, forcing him to put a heavy focus on fundraising. He was off the air for a few weeks and Baldwin spoke at the Democratic National Convention in Charlotte -- two reasons that could explain the swing in momentum. The Marquette poll's swing is pretty steep (and also shows President Obama leading Mitt Romney by a whopping 14 points), but even if the race is closer than it shows, it's clear the contest has tightened.

A race that does not appear to be tightening is the one in Virginia, where former Democratic National Committee Chairman Tim Kaine has built a lead in two new polls, one from the Washington Post and another from Quinnipiac/NYT/CBS. Kaine leads former senator George Allen (R) by 8 percentage points in the former survey and 7 in the latter. This comes after poll upon poll have shown a neck-and-neck race for much of the cycle.

In Massachusetts, where Sen. Scott Brown (R) seemed to do everything right this summer, four polls released this week showed Democrat Elizabeth Warren holding a slight lead. One pollster characterized the boost in support for Warren as a post-convention bounce.

It's not all bad news for Republicans. In Maine, where the National Republican Senatorial Committee has spent a six-figures on an ad buy, a new survey from automated pollster Public Policy Polling showed front-running independent former governor Angus King leading by just 8 points over Republican Charlie Summers, a much smaller lead than other surveys have shown.

If the race tightens further, Democratic outside groups will be in a tough spot there. Convinced King would caucus with the party if elected, Democrats have taken a hands-off approach when it comes to the race, all but ignoring Democratic nominee Cynthia Dill. Yet if the Democratic Senatorial Campaign Committee or Majority PAC runs ads for King, Republicans will immediately draw attention to it.

In another New England race once thought to rest firmly in Democratic hands, Republican Linda McMahon is holding her own against Rep. Chris Murphy (D-Conn.). Obama is likely to carry Connecticut, and if McMahon couldn't win in the 2010 GOP wave year, it's hard to imagine her winning this time. Still, her money and Murphy's recent string of tough headlines about him facing a foreclosure and failing to pay his rent make Connecticut a race to watch.

Republicans must net four seats to win the majority, a task which, while still possible, appears less likely than it once did. But as quickly as Democrats have seized the momentum, the GOP could get it right back.

If embattled Rep. Todd Akin (R) drops out in Missouri before the Sept. 25 deadline, Republicans can replace him and could once again have a good chance at beating Sen. Claire McCaskill (D). If the Maine race tightens, a surefire Democratic pickup may start to look shaky. And if the GOP builds leads in Massachusetts, Nevada and North Dakota, Democrats will have to look to less promising opportunities to make pickups in Indiana and Arizona. And they still have to play a lot of defense.

But those are a lot of ifs. This week at least, Senate Democrats have more to smile about.

Sean Sullivan has covered national politics for The Washington Post since 2012.

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